The passive optical network (PON) technology is a point to multi-point optical access technology. As shown in FIG. 1, a PON network is generally formed by an optical line terminal (OLT) 12, an optical splitter 14, an optical network unit (ONU) 16, and optical fibers connecting those devices. As a central office end device, an OLT connects to an optical splitter over a backbone optical fiber. The optical splitter connects to each ONU over an independent branch optical fiber. In a downstream direction, the optical splitter splits light, and sends downstream optical signals of the OLT 12 to all ONU 16s through branch optical fibers. In an upstream direction, the optical splitter 14 converges optical signals, converges the optical signals sent from ONU 16s, and sends the converged optical signals to OLT 12 through the backbone optical fiber.
To ensure that optical signals from each ONU reach the OLT at the same time, a ToD (Time of Day, Time of Day) transmission scheme is generally adopted for the existing PON. As shown in FIG. 2, the OLT 12 obtains the ToD from a certain time source 24 of a core network 22 (using a certain time synchronization mechanism of a packet network, for example, 1588v2). Then, the ONU 16 obtains the ToD from the OLT 12 to synchronize the time of the ONU 16. The inventor finds that in practical application, several ONUs on certain PONs have their respective high-precision time sources. If an OLT still obtains a ToD from a certain time source of the core network, multi-level transmission over a packet network is required. As a result, the precision of the ToD decreases.